Calculating Number of Sections
The following curves show the stopband frequencies normalized to the 3 dB bandwidth for filters with 2 to 8 sections. A ratio of stopband frequency to 3 dB bandwidth is used.
he curve given on the right shows an asymmetric frequency response resulting from the circuit used. Other schematics may be utilized to yield different attenuation characteristics (i.e. steeper on the high frequency side of the passband and shallower on the low side).
Example:
A BCSeries filter has a center frequency of 1000 MHz and a 3 dB bandwidth
of 50 MHz. Use the curve for 310% bandwidth filters. A stopband attenuation
of 40 dB is required at 760 MHz and 50 dB is required at 1140 MHz.
The
percentage bandwidth is 5%, calculated as follows:
For the first stopband
requirement:
Number of 3 dB bandwidths
from center frequency =
From the 310% bandwidth
attenuation curve, we find that a minimum of 3 sections is required.
The second stopband
requirement is:
Number of 3 dB bandwidths
from center frequency =
From the 1050% bandwidth
attenuation curve, we find that 4 sections minimum are required.
The greater number
of sections must be used to insure full specification compliance; therefore,
a 4 section should be used.
Insertion Loss Calculation:
Knowing the number of sections, center frequency and bandwidth of the filter, insertion loss may be calculated using the following formula:
Example: 4BC  1000/150S
1. Percentage BW = 50/1000 x 100 = 5%
2. LC from table = 2.5
3. Number of Sections (from P/N) = 4
4.
